The survey of planetary nebulae in Andromeda (M31) VI. Kinematics of M31 inner-halo substructures and comparison with major-merger simulation predictions

TL;DR

This paper investigates the kinematics of M31's inner-halo substructures through planetary nebulae velocities, supporting a recent major merger event that shaped the galaxy's current structure and halo composition.

Contribution

It provides observational evidence and simulation comparison confirming a recent major merger in M31, explaining the formation of substructures and halo characteristics.

Findings

PN velocities match red giant branch star data.

G1-clump is dynamically cold, indicating pre-merger disc material.

Inner halo is metal-rich, consistent with in-situ formation from a major merger.

Abstract

M31 has experienced a recent tumultuous merger history as evidenced from the many substructures that are still present in its inner halo, particularly the G1-Clump, NE- and W- shelves, and the Giant Stream (GS). We present planetary nebulae (PNe) line-of-sight velocity (LOSV) measurements covering the entire spatial extent of these four substructures. We further use predictions for the satellite and host stellar particle phase space distributions for a major merger (mass ratio = 1:4) simulation to help interpret the data. The measured PN LOSVs for the two shelves and GS are consistent with those from red giant branch stars. Their projected radius vs. LOSV phase space, links the formation of these substructures in a single unique event, consistent with a major merger. We find the G1-clump to be dynamically cold compared to the M31 disc ($\rm\sigma_{LOS, PN}=27$ km s$^{-1}$), consistent with pre-merger disc material. Such a structure can not form in a minor merger (mass ratio $\sim$1:20), and is therefore a smoking gun for the recent major merger event in M31. The simulation also predicts the formation of a predominantly in-situ halo from splashed-out pre-merger disc material, in qualitative agreement with observations of a metal-rich inner halo in M31. Juxtaposed with previous results for its discs, we conclude that M31 has had a recent (2.5 - 4 Gyr ago) `wet' major merger with the satellite falling along the GS, heating the pre-merger disc to form the M31 thicker disc, rebuilding the M31 thin disc, and creating the aforementioned inner-halo substructures.